1. Field of the Invention
The present invention relates to a method and apparatus for recording/reproducing holographic information.
2. Description of the Related Art
Technology for storing information by using a hologram has been widely used. In such technology, information is stored in an optical interference pattern shape in an inorganic crystal or a polymer material that is sensitive to light. Optical interference patterns are formed by using two coherent laser beams. In other words, interference patterns are formed when a reference light and a signal light having different paths interfere with each other, cause chemical or physical changes in a photosensitive information storage medium and are recorded thereon. A light that is similar to the reference light used to record information is irradiated on interference patterns recorded on the information storage medium so as to reproduce information from the recorded interference patterns. This causes diffraction due to the interference patterns and as such, the signal light is restored and information is reproduced.
Examples of technology for storing information by using holograms include a volume holography method by which information is recorded/reproduced in one page units by using volume holography and a micro-holography method by which information is recorded/reproduced in single bit units by using micro-holography. In the volume holography method, a large amount of information can be processed simultaneously. However, since an optical system must be very precisely adjusted, it is not easy to adopt the method in an apparatus for storing information for general consumers.
In the micro-holography method, two condensed light beams interfere with each other at a particular point of focus and thus form fine interference patterns, which are moved on a plane of an information storage medium, and a plurality of interference patterns are recorded on the information storage medium to form a plurality of recording layers. The recording layers are superimposed upon each other in a depth direction of the information storage medium to form a multi-layer structure so that information can be recorded three-dimensionally on the information storage medium.
In the micro-holography method, signal quality during reproduction is greatly affected by the diffraction efficiency of a recorded hologram. The diffraction efficiency of the recorded hologram is determined according to a difference in refractive indices of the information storage medium on which the hologram is recorded and which forms interference patterns, and according to the thickness of the recorded hologram. The diffraction efficiency of the recorded hologram is increased by an increase in the difference in refractive indices of the information storage medium and the thickness of the recorded hologram.
However, since the thickness of the recorded hologram is inversely proportional to the square of a numerical aperture (NA) of an optical pickup focusing optical system, when the NA is increased so as to increase a recording capacity, the thickness of the recorded hologram is lowered and as such, reflectivity is reduced by geometric progression. In other words, when the NA of the optical pickup focusing optical system is increased so as to increase the recording density, the diffraction efficiency of the recorded hologram is lowered and due to the lowered diffraction efficiency of a recording mark, it is difficult to obtain good signal quality during reproduction of the information storage medium. When the information storage medium having a threshold response is used so as to record a smaller mark than an optical minimization limit, signal quality due to the lowered diffraction efficiency of the recorded hologram may be further degraded.